The present invention relates to an automatic focusing device employed in, for example, a photographic camera, and more particularly to an automatic focusing device for automatically focusing on an object even when the object is moving.
In the field of an automatic focusing system for use in a photographic camera, there has been known a focus prediction AF (automatic focusing) system for focusing on an object-in-motion. The focus prediction AF system is a system which predicts a position that the object-in-motion will reach in a predetermined period of time, and controls the focusing lens of a camera so as to obtain an in-focus object image.
Specifically, such an automatic focusing system detects the distance to the object, i.e., distance to a portion of the object which is located at a rangefinding zone (AF detecting zone of a viewfinder field), or a defocus amount, and adjusts the focusing lens so as to focus on the object based on the measured distance or detected defocus amount. Therefore, the automatic focusing system cannot keep the lens focused on the object unless it detects the object within the rangefinding zone (AF detecting zone).
However, it is difficult to keep an object-in-motion within the area of the viewfinder field corresponding to the rangefinding zone (AF detecting zone) at all times, and another object may possibly enter the rangefinding zone (AF detecting zone). If an object-in-motion temporarily slips out of the rangefinding zone (AF detecting zone), the automatic focusing system is unable to remain focused on the object-in-motion anymore, and tends to focus on another object which is presently located in the rangefinding zone (AF detecting zone). Once another object is focused, the automatic focusing system may not re-focus on the object-in-motion immediately when the object-in-motion returns to the rangefinding zone (AF detecting zone). It would take a considerable period of time for the AF system to refocus on the object-in-motion. Thus, when successive exposures are being made, considerable number of pictures would be taken in an out-of-focus condition.